The present invention relates to a method of purifying bishydroxyalkyl terephthalate. More specifically, the present invention relates to a method of purifying crude bishydroxyalkyl terephthalate usable as a material for polyalkylene terephthalate such as recycled polyethylene terephthalate or polybutylene terephthalate.
Heretofore, polyalkylene terephthalate (hereinafter abbreviated as xe2x80x9cPET or the likexe2x80x9d) such as polyethylene terephthalate or polybutylene terephthalate is produced by causing dimethyl terephthalate or terephthalic acid to react with ethylene glycol or 1,4-butanediol to produce bishydroxyethyl terephthalate or bishydroxybutyl terephthalate (hereinafter abbreviated as xe2x80x9cBHET or the likexe2x80x9d) and condensing BHET or the like. In this case, since crude BHET or the like obtained from dimethyl terephthalate or terephthalic acid contains methanol or water as a by-product, crude BHET or the like is purified by distillation to remove methanol or water therefrom before condensation.
Meanwhile, in Japan, a container and package recycling law has been in effect since April in 1997, and demand for recycling PET or the like has been increasing. As methods for recycling PET or the like, a method in which recovered PET or the like is shredded, molten and processed into fibrous PET or the like which is used as fabric and a chemical recycle method in which recovered PET or the like is decomposed into raw material monomers (such as BHET or the like), which are recovered and PET or the like is synthesized again from the recovered monomers.
As a chemical recycle method for PET or the like, a method is known in which PET or the like is decomposed by solvolysis using a solvent such as an alcohol or glycol in the presence of a catalyst to recover monomers (such as BHET or the like) are recovered.
However, monomers (such as BHET or the like) recovered by the chemical recycle methods contain a variety of by-products produced by decomposition, colorants, adherent substances, and impurities such as ions contained in water used during the recycling process. Synthesis of PET or the like requires use of highly purified monomers. Since monomers recovered by the chemical recycle methods contain a large amount of impurities, these monomers as such cannot be used for the synthesis of PET or the like.
It is an object of the present invention is to provide a method of purifying bishydroxyalkyl terephthalate which is capable of obtaining highly purified monomers usable in synthesis of PET or the like.
The present invention is characterized in that crude bishydroxyalkyl terephthalate is treated with activated carbon and ion exchange resins to purify it.
Further, in another embodiment, the above treatment with activated carbon and ion exchange resins in the present invention comprises a solution passing step in which a solution containing crude bishydroxyalkyl terephthalate is passed through an activated carbon column filled with activated carbon and ion exchange resin columns filled with ion exchange resins, and an sweetening-off step in which ethylene glycol or an alcohol as a solvent is passed through each column after the solution passing step to displace bishydroxyalkyl terephthalate.
Further, in still another embodiment, the above treatment with the ion exchange resins in the present invention comprises a step of passing the solution through a single bed of a strongly acidic cation exchange resin and a step of passing the solution through a mixed bed of a strongly acidic cation exchange resin and a weakly basic anion exchange resin.
Further, in still another embodiment, the present invention has a filtration step which is carried out prior to the above treatment with activated carbon and ion exchange resin(s). The filtration is carried out at a temperature higher than a temperature at which the treatment with activated carbon and ion exchange resins is carried out by 5xc2x0 C. or lower.
Further, in still another embodiment, the present invention comprises a depolymerization step in which polyalkylene terephthalate is subjected to ester depolymerization to produce crude bishydroxyalkyl terephthalate and a purification step in which the obtained crude bishydroxyalkyl terephthalate is filtered and then purified and returns a solid separated by the filtration in the above purification step to the above depolymerization step.